This invention relates generally to surface wave integratable filters (SWIF's) and in particular to those utilizing multistrip couplers (MSC).
SWIF devices comprise piezoelectric substrates upon which transmitting and receiving transducer pairs are formed. The transducers, whether receiving or transmitting, are typically formed by pairs of electrically conductive comb-like structures having interleaved fingers. When a voltage is applied between the comb-like structures of a transmitting transducer, the piezoelectric material of the substrate's surface is stressed and deformed causing a conversion of electrical energy to mechanical energy in the form of acoustic surface waves which propagate across the medium surface to impinge the receiving transducer. A second energy conversion takes place at the receiving transducer in which the mechanical energy of the acoustic surface wave is reconverted to electrical energy developing a voltage between comb elements.
While a simple single transducer section, formed by two adjacent fingers of one comb element and the interleaved finger of the other, is capable of launching or receiving surface waves, in practice many transducer sections are combined in a more complex transducer structure. In addition, the configurations of the transducer comb elements both as to finger lengths and spacing are selected to provide the desired transfer characteristic. In multiple section transducers such as are generally used, the total launched or received wave is the cumulative effect of the individual transducer sections. Transducers define a single maximum energy or primary propagation and reception axes; most propagation occurs along this axis, however, a small but significant amount is radiated into or recorded from other directions. Generally the primary axis is in essentially orthogonal alignment with the transducer fingers while these other energy transfer directions are then non-orthogonal.
As is well known, the fingers within the transducer sections are spaced upon the substrate such that certain frequency signals (near the transducer synchronous frequency, or maximum response) produce surface waves which are cumulatively reinforced as they propagate across the transducer structure (therefore maximally coupled) while others are cumulatively cancelled (and therefore attenuated). The typical transfer function of a SWIF with fingers of equal length and spacing comprises a sin x/x characteristic in which a major passband and adjacent traps or minima are defined together with successively diminished secondary passbands (called frequency side lobes) symmetrically spaced about the major passband.
As mentioned above, surface waves propagating in directions other than the primary axis, have lower energy. Further, wave energy not propagating along the primary axis in general does not have the same cumulative characteristic of the entire transducer structure but rather a portion of it. This is more evident in the commonly known apodized structure composed of transducer fingers which are length weighted as a function of the inverse fourier transform of the desired transfer function. As a result, the frequency response along other than the primary axis is different from that along the primary axis. In SWIF devices having the transmitting and receiving transducers "in line," that is, both transducers having common primary axes, surface waves propagated along other than the primary axis are generally not troublesome. Such in-line devices are, however, plagued by the effects of bulk mode wave propagation. Transducers produce both surface waves (propagating at or near the medium surface) and bulk mode waves (propagating deep within the medium). These bulk mode waves are undesirable in surface wave devices because they often reflect off substrate boundaries and impinge the receiving transducer causing spurious responses which often has a different time delay and exhibits a completely different frequency response.
In sidestepping SWIF devices in which the transducers are laterally offset, their primary axes are parallel but not coincident, and an interposed multistrip coupler is positioned orthogonal to the primary axes. Surface waves propagating along the primary axis of the transmitting transducer in its channel are "converted" to surface waves propagating along the primary axis of the offset receiving transducer in its channel. The purpose of offsetting the transmitting and receiving transducers and sidestepping wave propagation is to avoid the deleterious effect on the transfer function caused by the undesired bulk waves. Simply stated, the sidestepping effect of the multistrip coupler assures that surface waves launched by the transmitting transducer which are offset by the multistrip coupler reach the receiving transducer while bulk waves, which are not offset by the multistrip coupler do not.
It is known, however, that the transfer function of sidestepping devices differs from that which is theoretically predicted. In particular both the upper and lower side lobes adjacent the primary passband of the transfer function are considerably greater than anticipated. Also the depth of adjacent traps is considerably reduced from the predicted value. When, for example, the SWIF described is used as a passband filter the increased response of the side lobes relative to the primary passband response results in less attenuation of signals outside the passband (out-of-band signals) reducing filter selectivity. Further the reduced trap depth can result in insufficient rejection of undesired signals. It should be noted at this point that the offset position of the transducer generally results in some direct coupling of energy propagating along other than primary axis. Experiment has shown that one of the mechanisms contributing to transfer function disparity results from this direct coupling of energy via waves which impinge the multistrip coupler at an angle other than the 90.degree. angle required for conversion and, therefore, are not sidestepped but pass directly through the coupler impinging the receiving transducer. As mentioned, these spurious direct coupled waves do not have the same transfer function, and, therefore, their contribution to the coupled signal alters the effective transfer function.